Plain English Meaning
A shape that lets air slip past it easily instead of fighting against it. Think of a smooth, tapered form that air can flow around without breaking up into messy swirls.
Full Definition
An object whose shape is designed to allow air (or another fluid) to flow around it smoothly, with minimal turbulence and drag. A streamlined shape typically tapers from a rounded leading edge to a narrow trailing edge, allowing the airflow to follow the surface and rejoin cleanly behind it.
Context Anchor
Seen in basic aerodynamics when explaining how air moves around a wing or other aircraft part.
Derivation
From 'streamline' — the path a single particle of air follows as it flows past an object. An object is 'streamlined' when its shape lets those flow lines smooth and orderly instead of breaking into turbulence.